The present invention generally pertains to the stackability of hollow products and is particularly directed to an improvement in the stackability of hollow products of the type that includes at least a section of a generally conically contoured sidewall having a varying wall thickness and a number of longitudinal folds of alternating ridges and furrows. The longitudinal folds enhance the stiffness of such hollow stackable products and the variation in wall thickness may reduce the required injection pressure and clamp force when the product is manufactured by an injection molding process.
A prior art product of this type, as described in U.S. Pat. No. 3,194,468, is illustrated in FIGS. 1 and 2, which show three cups 10 stacked together with one cup stacked within a second cup which is stacked within a third cup. Each cup 10 has a generally conically contoured sidewall 12 having a number of longitudinal folds 14 of alternating ridges 16 and furrows 18. Each fold 14 includes a first side strip 20 laterally extending from the top of a ridge 16 to the bottom of an adjacent furrow 18 and a second side strip 22 laterally extending from the bottom of the furrow 18 to the top of the next adjacent ridge 16. The first side strips 20 have a different lateral orientation than the second side strips 22. The wall thickness at the bottom of the furrows 18 is greater than the wall thickness of the first and second side strips 20, 22.
When the cups 10 are stacked within one another, as shown in FIGS. 1 and 2, the cups 10 contact each other at the furrows 18, which have the greater wall thickness, such that there is a substantial space 24 between the first side strips 20 and/or between the second side strips 22 of the adjacent cups 10. Consequently, the stacking height of the cups 10 is greater than it would be if there was not a substantial space 24 between the first side strips 20 and/or between the second side strips 22 of the adjacent stacked cups 10. The stacking height of a product is the difference in height between the top of the product when stacked within a like product and the top of the like product. The stacking height of a product is approximately inversely proportional to the quantity of the product that can be stacked within a container of a given size.